This invention relates to the melt preparation of polycarbonate using a computerized process model to provide an optimal set of reaction conditions and reaction stoichiometry to effect the conversion of reactant diaryl carbonate and at least one dihydroxy aromatic compound to product polycarbonate having a predetermined molecular weight and level of endcapping.
Increasingly, polycarbonate is being prepared by the melt reaction of a diaryl carbonate with a dihydroxy aromatic compound in the presence of a transesterification catalyst, such as sodium hydroxide. In this xe2x80x9cmeltxe2x80x9d process reactants are introduced into a reactor capable of stirring a viscous polycarbonate melt at temperatures in excess of 300xc2x0 C. Typically, the reaction is run at reduced pressure to facilitate the removal of by-product aromatic hydroxy compound formed as the diaryl carbonate reacts with the dihydroxy aromatic compound and growing polymer chains. It is frequently desirable to prepare polycarbonates having a high level of endcapped polymer chain ends in order to promote polymer stability and to reduce the tendency to accumulate a static charge of molded articles prepared from the polycarbonate. Thus, it is desirable to maximize the percentage of polymer chains terminating with aryloxy groups, the xe2x80x9cendcappedxe2x80x9d chains, while minimizing the percentage of polymer chains terminating with hydroxyl groups.
According to the principles of condensation polymerization established by Flory and others, it should be possible to prepare polycarbonate, for example bisphenol A polycarbonate, in which all of the chains terminate in aryloxy groups or hydroxyl groups simply by adjusting the ratio of diaryl carbonate to dihydroxy aromatic compound. In a condensation polymerization taking place in a melt reaction between diphenyl carbonate and bisphenol A, for example, the use of excess diphenyl carbonate, wherein the molar ratio of diphenyl carbonate to bisphenol A is greater than 1.0, might be expected to provide a polycarbonate in which all of the polymer chains terminate in phenoxy groups, the molecular weight of said polycarbonate being determined by the extent to which the molar ratio of diphenyl carbonate to bisphenol A exceeds 1.0. In practice, however, it is found difficult if not impossible to achieve the complete endcapping of polycarbonate prepared in this manner. It is found that the use of excess diaryl carbonate in the melt reaction of a diaryl carbonate with an dihydroxy aromatic compound slows the rate of polymerization, and requires the use of higher catalyst levels and higher reaction temperatures in order to achieve high molecular weight polycarbonate. The high molecular weight polycarbonate so prepared still contains a significant percentage chain ends terminating in hydroxy groups.
In practice, the melt polymerization reaction of diaryl carbonates with dihydroxy aromatic compounds typically involves contacting the diaryl carbonate with the dihydroxy aromatic compound in an amount such that the molar ratio of diaryl carbonate to dihydroxy aromatic compound is initially in a range between about 1.05 and about 1.2. High molecular weight polymer is obtained as the reaction proceeds and by-product hydroxy aromatic compound as well as a portion of the excess diaryl carbonate is removed from the reaction mixture during the polymerization reaction. The xe2x80x9ceffective molar ratioxe2x80x9d of diaryl carbonate to dihydroxy aromatic compound, the molar ratio of carbonate groups to structural units derived from the dihydroxy aromatic compound in the product polycarbonate, is preferably in a range between about 1.01 and about 1.03. When the effective molar ratio of diaryl carbonate to dihydroxy aromatic compound is in a range between about 1.01 and about 1.03, polycarbonate having a high number average molecular weight, Mn in a range between about 8,000 and about 28,000 Daltons, is obtained. Frequently, however, in order to achieve a product polycarbonate having an effective molar ratio of diaryl carbonate to dihydroxy aromatic compound in a range between about 1.01 and about 1.03, a relatively high starting molar ratio of diaryl carbonate to dihydroxy aromatic compound must be employed and the polymerization mixture must be heated for a prolonged period of time in order to overcome low reaction rates, which are the result of lower concentrations of reactive hydroxyl groups, and to remove at least some of the excess diaryl carbonate employed. During such prolonged heating undesired rearrangement products, such as a Fries rearrangement product, may be formed.
The effective molar ratio of diaryl carbonate to dihydroxy aromatic compound actually observed is dependent upon the various reaction parameters employed, such as reaction time, temperature and pressure as well as the starting molar ratio of the reactants, and catalyst concentration and catalyst identity. Moreover, the effective molar ratio of diaryl carbonate to dihydroxy aromatic compound obtained in the product polycarbonate is affected by those characteristics of the reactor employed which affect the rates of removal of by-product aromatic hydroxy compound and excess diaryl carbonate, said characteristics of the reactor employed include the shape and geometry of moving internal components of the reactor, such as a stirrer, the agitation rate, and the reflux ratio operative during distillative removal of by-product hydroxy aromatic compound.
It would be a significant advantage to prepare polycarbonate by a melt polymerization method in which the molecular weight of the product polycarbonate and the level of endcapping could be selected, and thereafter, using a process model, reaction parameters could be determined which would most efficiently provide the product polycarbonate having the predetermined level of endcapping and the predetermined molecular weight. It would be particularly desirable to determine, for a given reactor operating under a given set of reactor conditions, said reactor conditions comprising reaction temperature, reaction pressure, reaction time and reaction catalyst concentration and catalyst identity, what the proper initial molar ratio of diaryl carbonate to dihydroxy aromatic compound should be in order to obtain a product polycarbonate having an effective molar ratio of between about 1.01 to about 1.03. Moreover, it would be desirable to employ the same process model to allow the minimization of Fries product in the product polycarbonate.
The present invention provides a method for making a polycarbonate comprising structural units derived from at least one dihydroxy aromatic compound and at least one diaryl carbonate, said method comprising the following steps:
Step (I) selecting a desired molecular weight and percent endcapping of the product polycarbonate;
Step (II) consulting a computerized process model to obtain a starting molar ratio of diaryl carbonate to dihydroxy aromatic compound;
Step (III) charging an amount of dihydroxy aromatic compound and diaryl carbonate in the molar ratio indicated by the process model to a reactor to form a reaction mixture, said reactor being an element of said process model; and
Step (IV) heating at a temperature and pressure for a time period indicated by the process model in the presence of a transesterification catalyst while distilling from the reactor a mixture comprising by-product hydroxy aromatic compound and diaryl carbonate to provide a polycarbonate having an effective molar ratio of diaryl carbonate to dihydroxy aromatic compound and having the desired molecular weight and percent endcapping.
The present invention further relates to a method for making a polycarbonate which minimizes the amount of Fries product present in the product polycarbonate.